1. Field of the Invention
The present invention relates to a semiconductor thin film transistor (TFT), and more particularly to a TFT which allows current to flow in a vertical direction with respect to the surface of a semiconductor thin film.
2. Description of the Related Art
Generally, a thin film transistor (TFT) is defined as a field effect transistor (FET) made by forming a semiconductor thin film on an insulating substrate. Like the FET, the TFT is a device with three terminals, i.e., gate, source, and drain, and is mainly used as a switch. Although it is used in a sensor, a memory device, and an optical device, the TFT is mainly used as a pixel-switching device for an active matrix flat panel display. In particular, the TFT is widely used as a pixel-switching device or a current-driving device for a liquid crystal display or an organic electroluminescent display.
A TFT with an amorphous silicon semiconductor thin film generally has a horizontal structure as shown in FIG. 1. Referring to FIG. 1, a first electrode 20, which serves as a gate, is formed on a substrate 10 and a dielectric thin film 30 is formed on the first electrode 20. An amorphous silicon thin film 60 is formed on the dielectric thin film 30. A second electrode 40 and a third electrode 50, which serve as a source and a drain, respectively, are formed on the amorphous silicon thin film 60 in such a way as to be spaced apart from each other. Various modifications in the positions and shapes of the dielectric thin film 30, the amorphous silicon thin film 60, and the first, second, and third electrodes 20, 40, and 50 may be made. However, every type of TFT has a common on/off switching operation, that is, current flows between the second electrode 40 and the third electrode 50, and an electric field generated by a voltage applied to the first electrode 20 acts on the current perpendicularly to the current direction.
Here, provided that the thickness and width of a channel formed in the amorphous silicon thin film 60 are designated t and L, respectively, current flows through the channel with a cross sectional area of t×L. Generally, since the thickness t is in the range of several nanometers to several tens of nanometers, a change in the thickness t insignificantly contributes to an increase in the total current. Therefore, current flows through the channel with a very small cross sectional area adjacent to the interface between the amorphous silicon thin film 60 and the dielectric thin film 30, which makes it difficult to increase the switching speed of a TFT.